Control rods for a nuclear reactor are generally cruciform in shape, with each wing of the control rod containing neutron-absorbing material. One such material is hafnium. The hafnium rod is typically disposed within an absorber tube and connected to a stainless steel plug at one end of the absorber rod, the plug being welded to the end of the absorber tube. The end plug has a projecting male threaded portion with a thread relief between the plug head and the male threads. The complementary end of the hafnium absorber rod about the female threads has a bore provided with female threads for threadedly receiving the male threaded portion of the end plug. Typically, the annulus about the end of the hafnium rod is provided with a lateral opening or cross-hole whereby the end plug portion screwed into the hafnium rod can be staked in place. The opposite end of the hafnium rod is otherwise unsecured to the absorber tube and is spaced from the opposite end plug of the tube to provide a gap allowing for high dynamic loads due to rapid acceleration/deceleration.
It will be appreciated that the control rods can be subjected to very high acceleration/deceleration loads, e.g., during a failed buffer scram event. Such loadings may be up to 166 inches per second. These high acceleration/deceleration loads may cause a failure in the absorber tube and particularly in the connection between the hafnium rod and the end plug. Four areas of the connection are particularly highly stressed and therefore subject to failure during such loadings: (1) the male threads on the end plugs; (2) the female threads on the hafnium rod end; (3) the thread relief on the end plug; and (4) the thin walls of the annulus about the threaded bore and bore extension in the hafnium rod. Consequently, these high acceleration/deceleration forces can fracture the connection between the hafnium rod and the absorber tube through the end plug at various locations. The potential for fracture in these high stress areas will be particularly appreciated when it is recognized that the thread relief between the plug head and threads is 0.100 inch in diameter. Also, when the hafnium rod is drilled to receive the end plug threads, the resulting annulus cross-section is 0.034 inch thick. The annulus is additionally weakened in the area of the stake hole which is 0.06 inches in diameter. Consequently, large accelerations/decelerations may cause a fracture at the connection of the hafnium absorber rod and the end plug.